In systems for the detection of point source signals in the presence of background clutter, interference or other noise, there is a common problem whether the system involves sonar, radar, infrared or video detection techniques. In such detection systems, the presence of a point source signal commonly is determined by comparing the level of the received signal against a "threshold" value which may be either fixed or adaptively adjusted in response to one or more system operating parameters. The value of this threshold level desirably is set as low as possible, to enhance system sensitivity and thus enable detection of weak signals, as from distant sources. If set too low, however, the system will respond to clutter and other background noise, which may overload the post-detection processor with too much data and result in an objectionably high false alarm rate (FAR).
To enable the lowest possible detection threshold while maintaining a satisfactory FAR, it is common to precede the thresholding comparator with background normalization means which removes or minimizes the unwanted sources and background clutter that is present around the point source being detected, thereby providing a spatially filtered output which indicates the signal intensity from the desired point source with minimal signal contributions from all other sources around the point source.
One such filter as generally implemented includes means for deriving a spatially filtered signal by calculating the average or mean signal level of the background surrounding the desired point source and subtracting this average background signal level from the point source signal. The spatially filtered signal then is compared with a threshold signal to determine if a point source signal is present. In this manner spatial filtering accentuates the difference between the point source signal and background clutter, which is basically the direct current (D.C.) content and which is eliminated through such subtraction.
The prior art spatial filter systems include independent horizontal and vertical high pass filters to remove the low frequency content of the scene. Every point source in the system field of view is treated the same in that the surrounding elements are multiplied by fixed values and summed with the original value. The output for any element thus contains portions of surrounding elements. The result is artifacts in the output which may also become detections. It is, therefore, desirable to eliminate these artifacts around the point source that are generally produced by the presently available filter systems.
It is accordingly an object of the present invention to provide a point source detection system which minimizes or eliminates the effect of artifacts located around the point source that are generally produced by current filtering methods. It is another object of the present invention to provide a spatial filter which minimizes false detections by minimizing the signals from all sources other than the desired point sources.